Recently there has been a demand for magnetic recording media capable of high-density recording. A magnetic material that meets this demand should have high coercive force (Hc), high saturation magnetization (.sigma.s), and low-noise performance. In order to fulfill these requirements, several methods have been proposed.
For instance, high coercive force (Hc) is achieved by increasing the length/diameter ratio of needle crystals of .alpha.-FeOOH (which is a raw material of magnetic iron oxide) and calcining .alpha.-FeOOH without breaking the needle shape of the crystals, or by incorporating magnetic iron oxide with cobalt.
The most effective way of reducing noise of a magnetic material itself is to make the particles of a magnetic material fine (i.e., small particle size). This method, however, has a disadvantage, in that, as the particles are made finer, the unit of magnetization becomes small and the magnetization becomes thermally unstable. Magnetic tape made of such a magnetic material is poor in print-through property (S/P). The relationship between the crystalline particle size and the print-through property is graphically shown in FIG. 1. According to the conventional technology, cobalt-modified iron oxide (marked with circles) becomes poor in print-through property (S/P) as the crystalline particle size is made finer, and it has been impossible to improve it beyond the limit indicated by the broken line. In addition, making the particles of a magnetic material finer has another disadvantage, viz., decreasing the saturation magentization (.sigma.s).
There are several known processes for producing cobalt-modified ferromagnetic iron oxide.
According to a first category of processes, cobalt-modified ferromagnetic iron oxide is produced by causing iron oxide to form a solid solution with cobalt. Such processes are described in U.S. Pat. Nos. 3,117,933 and 3,671,435, Japanese Patent Publication Nos. 6538/19566, 4264/1974, 27719/1966 (counterpart of U.S. Pat. No. 3,573,980), 15759/1973, 10994/1973, and 6113/1967, and Japanese Patent Application (OPI) No. 101599/1973 (the term "OPI" as used herein refers to "published unexamined Japanese Patent Application"). The cobalt-containing iron oxide produced according to these processes, however, suffers from some disadvantages when applied to magnetic tape or magnetic recording media. That is, it is unstable to pressure and heat and the magnetic signals recorded thereon become weak with time and undergo considerable print-through.
According to a second category of processes, cobalt-modified ferromagnetic iron oxide is produced by coating or growing a cobalt compound layer or a cobalt-ferrite layer on the surface of magnetic iron oxide powder containing no cobalt (in the form of solid solution). Such processes are described in Japanese Patent Application (OPI) Nos. 108599/1974, 37667/1975, 37668/1975, 108599/1974, 37667/1975, 37668/1975, 82076/1975, 5497/1977, 5498/1977, and 129894/1978; Japanese Patent Publication No. 49475/1974; and West German Patent Application (OLS) No. 2,905,352. The magnetic iron oxide powder produced according to these processes is more stable to pressure and heat and improved in print-through property as compared with the powder produced according to the first category of processes. Nevertheless, it is impossible for these processes to provide a magnetic iron oxide powder which exhibits performance beyond the broken line in FIG. 1.
According to the conventional technology, .gamma.-Fe.sub.2 O.sub.3 which is modified with cobalt is usually produced by (1) dehydrating .alpha.-FeOOH at 300.degree. to 700.degree. C. to provide .alpha.-Fe.sub.2 O.sub.3, (2) reducing the .alpha.-Fe.sub.2 O.sub.3 in a reducing gas at 300.degree. to 400.degree. C. to provide Fe.sub.3 O.sub.4, and (3) oxidizing the Fe.sub.3 O.sub.4 at a low temperature of from 200.degree. to 300.degree. C. The final oxidizing step is accomplished at a comparatively low temperature because it has been considered that .gamma.-Fe.sub.2 O.sub.3 is partly converted, irreversibly, into .alpha.-Fe.sub.2 O.sub.3, which is a non-magnetic stable material, when subjected to a high temperature. This leads to a decrease in magnetism.